1. Field of the Invention
The present invention generally relates to a gear of a resin for use in power transmission devices for precision instruments, such as color copying machines, color printers and video tape recorders, various automotive parts, industrial equipments and so forth. More specifically, the invention relates to a resin gear which is formed by an injection molding. The present invention also relates to an injection molded resin gear and an injection molded resin rotating body, such as an injection molded resin roller or an injection molded resin pulley. Moreover, the present invention relates to an injection molded article which is used as a dynamic rotating body, such as an injection molded resin gear or an injection molded resin roller, or a static shaft supporting member.
2. Description of the Prior Art
For example, tandem type color copying machines and color printers are designed to sequentially carry out steps of forming images of black (K), magenta (M), cyanogen (C) and yellow (Y) in a direction in which a transfer material, such as a copy paper, is carried, to sequentially superpose and transfer images of different colors to the transfer material to print a desired color image on the transfer material. In such color copying machines and color printers, an image forming unit for each color (K, M, C, Y) is connected to a motor via gears to be operated by the driving force of the motor. Therefore, in order to enable a high quality color print, it is required to precisely operate the image forming unit for each color (K, M, C, Y) to precisely transfer each color image to the transfer material. For that reason, the precision of gears for transmitting the rotation of the motor to the image forming unit is important. That is, if the precision of the gears is bad, the rotation of the motor is not precisely transmitted to the image forming unit, so that operation failure, such as irregular rotation, is caused in the image forming unit, thereby causing print failure, such as color deviation.
In addition, users have requested to reduce the size, weight and price of the above described color copying machines and color printers and to reduce operation noises thereof.
Therefore, in recent years, the above described color copying machines and color printers have widely used injection molded resin gears, which can be more precise than cut metal gears and which can more greatly reduce weight and operation noises than the metal gears, as gears for transmitting the rotation of a motor to an image forming unit.
FIG. 17 shows an example of a conventional injection molded resin gear. The injection molded resin gear 31 shown in FIG. 17 is formed by injecting a molten resin into a cavity from three pin point gates 35, which are arranged around the center of the side of a web 34 for connecting a rim 32 and a hub 33 (see, e.g., Japanese Patent Laid-Open No. 49-36755).
FIG. 18 shows another example of a conventional injection molded resin gear. The injection molded resin gear 41 shown in FIG. 18 is formed by applying a method for forming an impeller for a circumferential current pump, which is disclosed in Japanese Patent Laid-Open No. 2001-123986. This gear 41 is formed by injecting a resin from a ring-shaped gate 42 which allows the resin to flow outwardly and inwardly in radial directions of a web 43 so as to prevent the production of weld lines.
FIG. 19 shows a further example of a conventional injection molded resin gear. The injection molded resin gear 51 shown in FIG. 19 is formed by injecting a resin from a disk gate 52 which allows the resin to radially flow from a hub 53 toward a web 54 so as to effectively prevent the production of weld lines (see, e.g., Japanese Patent Laid-Open No. 2001-353753).
However, if the resin is injected into the cavity from the three pin point gates 35 as shown in FIG. 17, the resin injected from each pin point gate 35 flows into each other in substantially middle portions between adjacent pin point gates 35 and 35 as shown by arrows to produce weld lines 36. If the weld lines 36 are produced in an injection molded article (the injection molded resin gear 31), there are some cases where the weld lines 36 do not only have a bad influence on the precision of the tooth profile, but they also have a bad influence on the strength of the gear. Therefore, the injection molded resin gear 31 shown in FIG. 17 is unsuited for a power transmission device for a color copying machine or the like.
In addition, in the injection molded resin gear 41 formed by using the ring-shaped gate 42 as shown in FIG. 18, the amount of useless material used in the gate and runner is larger than that in the above described injection molded resin gear 31 using the pin point gates 35, so that there is a problem in that production costs are high.
Moreover, the disk gate 52 shown in FIG. 19 is arranged at the center of the injection molded resin gear 51. As a result, an axial die 56 for forming an axial hole 55 can not securely be supported on the side of a top die 57, and the axial die 56 is easily moved by the injection pressure, so that there is a problem in that the axial hole 55 of the injection molded resin gear 51 is easy to be eccentric to have a bad influence on the results of measurement of run out of a tip circle.